A membrane circuit board includes a first flexible circuit board, a second flexible circuit board and a nanomaterial layer. The nanomaterial layer includes plural polymeric structures. The nanomaterial layer is formed on the first flexible circuit board, the second flexible circuit board and/or a junction region between the edge of the first flexible circuit board and the second flexible circuit board to prevent at least one of the upper metallic conductor line and the lower metallic conductor line from contacting with a specified chemical element. Consequently, the conductive impedance of the upper metallic conductor line and the lower metallic conductor line is not affected by the specified chemical element. Moreover, the present invention also provides a keyboard device with the membrane circuit board.